Electrodynamic loud-speaker



Dec; 6, 1949 L. W. MURKHAM ELECTRODYNAMI C. LOUDSPEAKER Fiied June 19,:|947

Patented Dec. 6, 1949 UNITED STATES P OFFICE ELECTRGDYNAMIC LOUD-SPEAKERland, a British company Application June, 19, 1947,- Seria-l No.`155,699 In Great Britain December 4, -1-941 Section 1, Public Law 690,August 8,1946 Paten-t expires DecemberI 4, 1961 4 claims. (ci.17e-115.5)

This invention relates to vloud speakers or the like of the moving-coiltype in which the diaphragm is vibrated by means of a coil which carriesthe speech current and is located in the magnetic iield set up by amagnet. In loud speakers of this type the moving-coil is usuallyconnected to the diaphragm by being wound on a former in the shape of ashort cylindrical shell one edge of which is attached concentrically 'tothe diaphragm, and the magnetic iield is usually the eld set up acrossthe small annular air gap between the o-uter cylindrical surface of aninner pole piece and the inner cylindrical surface oi a surroundingouter pole piece, the said 'former with the coil in it projecting intothis air gap. Again in such loud speakers, it is usual for the outerpole piece to take the form of a plate with a circular concentric holein it Vforming the inner cylindrical surface aforesaid, and for theinner pole piece to be extended rearwardly in the direction away fromthe diaphragm, the magnetic circuit being completed by means of amore-or-less pot-shaped structure whose bottom' joins the rear end ofthe inner pole piece and whose rim or edge joins the back surface of theplate forming the outer pole piece.

In loud speakers of this kind, in order to obtain maximum acousticefficiency, it is necessary lto work with the smallest possibleclearances between the coil and former on the one hand and the surfacesforming the annular air .gap on the other, and diiculty has beenheretofore experienced in centering the-coil correctly in the gap andrendering the centering immune from disturbance by mechanical shock.

The invention has among its objects the provision ci improved meanswhereby this centering may be effected and securely maintained, theassembly of the parts in correct centre being entirely automatic.Another object is that of simplifying the magnet construction. Anotherobject is the provision of improved means for preventing the deleteriouseffects of vair cushioning at the back of the diaphragm. Y

The invention comprises the arrangement that both the diaphragm unitand' the plate forming the outer pole .piece are centred by tting themaccurately in a circular recess formed say in the back surface of thefront plate of the outer housing which houses said diaphragm and themagnet.

The invention also comprises in constructions in which the inner polepiece is centred relative to the plate forming 'the outer' p'cile pieceby means of a non'ferrous 'bush mounted on 'the back o'f said plate, inwhich bush the rearwardly extending part of said inner pole vpiece Veryaccurately fits, eiecting the attachment of the nonferrous bush to therear surface of said plate by heat springing, a counter-sunk' recess inthe rear surface oi said plate being shrunk, by cooling, on to saidbush.

The invention also comprises, in constructions in which the inner pole.piece is centred relative to the plate formingl 'the outer polepiece bymeans of a nonierrous bush mounted 0n the back of said plate, in whichbush the rearwardly extending part of said inner pole Vpiece Veryaccurately nts, the arrangement that holes or cut away portions areformed in 'the bush .for the purpose of reducing cushioning behind thediaphragm.

Other features of the invention will appear hereinafter.

In order that the invention may be the more clearly understood a loudspeaker in accordance therewith will now be described, reference beingmade to the accompanying drawings wherein:

Figure 1 is a sectional side elevation of ythe vwhole assembly;

Figure 2 is a plan of one of the parts of Figure 1;

Figure 3 shows a construction which may be employed in the bottom of theouter container I3 of Figure 1. A

Referring to Figure 1 the diaphragm I and the magnet are housed in acylindrical container of nonmagnetic material, whose lfront end 2, whichis just in front of the diaphragm, has a central opening 3 thereinthrough which the sound waves from the diaphragm are adapted to pass.This iront end 4has formed in its rear face a shallow circular recess 4,and both the diaphragm AI and therefore the movingl coil la and thecircular plate 5 forming the outer pole piece of vthe magnet, arecentred by fitting closely within this shallow recess. Thus thediaphragm, which is of the usual .paper-.like material or of thematerial generally used 4for this purpose, -is framed in a flat ringframing t of some stiffer material which ts very accurately in thisshallow circular recess and the said circular plate 5 forming the outerpole Lpiece of the magnet ts accurately behind the diaphragm 'in thesame circular recess thereby clampi-ng the `frame 6 in place andensuring that the said outer pole piece and the moving coil la shallboth be accurately centred. The clamping action of the said circularplate is against the ring 'framing of the diaphragm which is slightlythicker `than the diaphragm proper. 'This fact, added to 'the fact thatthe 3 front face of said outer pole piece is chamfered slightly towardsits central opening as shown, ensures plenty of room for the diaphragmto vibrate in. In like manner the shallow recess 4 in the front end 2 ofthe housing is formed to enable the diaphragm to vibrate freely.

The inner pole piece 1 is centred, relative to"Y this circular plate 5forming the outer pole piece by means of a non ferrous bush (see alsoFigure 2) mounted on the rear surface 'of said circular plate, in whichbush the rearwardly ex-.

than the outside diameter of the disc forming the bush 8. Said circularplate 5 is heated until its expansion permits the bush 8 to be tted intothis cylindrical recess and, upon cooling, the bush in said recess.

tending part of said inner pole piece very accurately fits. Thus, themoving coil I and the outer pole piece 5 both being accurately centredby the shallow circular recess 4 in the front plate 2 of the container,and the inner pole piece I being accurately centred by the nonferrousbush 8 on the rear of the outer pole piece 5, the moving coil Ia will beaccurately centred with respect to the annular clearance as a whole, andthis centering cannot become displaced.

It will be seen that the machining of the shallow circular recess 4, theinner and outer ma- `vrearwardly from said cylindrical recess.

will be gripped tightly and secured permanently The accurate machining,both of the inner surface of the bush and of the inner and outersurfaces of the circular magnet plate 5 may be effected after this heatspringing operation. It will be seen that the bush 8 projects Themachining of the inner surface of said bush is chining of the plate 5forming the outer pole piece, Y,

tively to said outer periphery m'ust all be very accurate. Once theseparts have their surfaces yaccurately formed their assemblyautomatically becomes correct and cannot thereafter become incorrect.

The nonferrous bush 8 also serves to locate the inner pole piece Iaxially relative to the outer pole piece. Thus the inner pole piece isformed with a small shoulder 9 which engages with the bush to limit therearward axial movement thereof relative to said outer pole piece, and,in construction, the rear end of the inner pole piece is secured, bymeans of a single central screw I, to the bottom II of the pot portionof the mag net, the dimensions being such that, in screwing up thescrew, this shoulder, engaging the bush,

strains the bush slightly, thereby ensuring that Athe circular plate 5forming the outer pole piece is pulled hard down on to the edge or rimof the wall I2 of the pot, which wall constitutes the permanent magnet.The said bottom II of the pot is made quite separate from thecylindrical 4wall I2 thereof, and all four parts viz. the outer polepiece 5 and bush 8, the inner pole piece l, the pot bottom II and thecylindrical pot wall I2 are secured together by means of the one screwI0 aforesaid passing through the pot bottom II into the rear of theinner pole piece '1. The actual pole surface of the inner pole piece 'Iopposite to the corresponding surface of the outer pole piece 5 is ofgreater diameter than the remainder of said inner pole piece. The saidshoulder 9 which abuts the nonferrous bush 8 is very small and is somedistance behind the said pole surface of increased diameter.

It will be appreciated that the parts of the -magnet must be machinedaccurately to ensure vthat the rear end of the inner pole piece I abutsfirmly against the rear plate II of the magnet, land at the same time,the cylindrical wall I2 of the magnet is clamped tightly between saidrear plate and the front plate 5 forming the outer pole piece. Theintensely concentrated field of the lmagnet adds to the mechanicalstrength.

The attachment of the nonferrous bush S to the rear surface of thecircular magnet plate 5 `constituting the outer'pole piece is preferablysuch that the forward portion thereof is of the same diameter as theinner or pole surface of the circular plate, the rear portion being ofsmaller diameter and forming the bearing surface for the irmer polepiece 7. In this way also a shoulder for engaging the aforesaid smallshoulder 9 on the inner pole piece is formed.

It will thus be seen that the nonferrous bush 8 is solidly attached tothe magnet plate 5 without the use of any screws and n such a way thatconcentricity between the pole surface of the magnet plate on the onehand and the bearing surface of the bush, and therefore the pole surfaceof the inner pole piece 7, on the other hand. can never be disturbed.

When the one central screw I0 is screwed up, the bush 8`may be withdrawnsay .015 of an inch from the cylindrical recess owing to the fact thatthe error in the length of the inner pole piece I up to the smallshoulder 9 may be as much as that amount. This will cause no damage andwill not in any way weaken the magnet structure in respect of itslateral or centralising strength. Once the magnetising is effected themagnetic Vforce pulling the two magnetic plates 5 and II towards oneanother is so strong as to aiord the bulk of the longitudinal strengthof the magnet structure.

The cylindrical housing of nonmagnetic material consists of theaforesaid front end 2 and an integral pot-shaped portion I3. When themagnet as a whole has been assembled, the assembly of the remainder iseffected by locating rst the diaphragm I and then the outer pole piece 5of the magnet, in the shallow circular recess 4 in the rear face of thefront end 2 of the outer container, then placing the pot portion I3 ofthe outer container in position against the said front end and thenscrewing said front end to said pot portion by means of a number ofscrews I6 screwed into thickened portions of the wall of said potportion. The dimensions are such that this presses the outer pole piece5 of the magnet against the ring framing 6 of the diaphragm and'completes the clamping action as described.

To obviate the disadvantages of air cushioning at the rear of thediaphragm I, which tends to reduce the bass output and give articialaccentuation elsewhere, a number of, say four, holes I5 are drilledthrough the circular plate 5 form- Ving the outer pole piece, say in thechamfered area as close as possible to the gap without interfering withthe magnetic path. (These holes must also pass through the nonferrousbush 8.) This prevents air cushioning behind the part of the diaphragmoutside of the coil Ia. Additional holes or cut away portions I6 arealso formed in the bush 8 in line with the annular clearance. i. e.

in line with the coil Ia, thus preventing serious cushioning behind thepart of the diaphragm inside of the coil as air is permitted to escapethrough the annular gap, though it is true that this narrow gap doesoffer some resistance. Also, to allow air to escape from within themagnet pot a number of holes I1 are formed in the rear plate II of themagnet structure. Alternatively the air could be arranged to escape pastthe rear plate of the magnet structure, the latter being spaced byspacers from the cylindrical Wall I2, but this is not usually sosatisfactory. The outer` container is very considerably larger than themagnet and thus the trapping of air therein is not deleterious. Toenable the container to be thus larger than the magnet the latter abutsagainst pedestals I8 extending from the bottom of the container pot.

Rubber pads or spring washers or plastic rings I9 or other form ofcompressible packing may be interposed between the plate I I and thepedestals I8 or rear of the housing. Similar packing 20 may also form awater tight joint between the front and the pot-portion of the outerhousing.

In cases where the loud speaker will not have to be installed out ofdoors the outer container could be formed with openings to theatmosphere. Where the loud speaker may have to be installed out of doorsand openings to atmosphere are required, the arrangement illustrated inFig-ure 3 may be employed. In this arrangement the bottom of the potportion I3 has a central hole 2| formed in it and this hole is coveredby means of a cap 22 which is spaced away from the bottom of the potportion I3 by means of lugs 23 integral with said pot portion I3. Aircan flow freely out of the hole 2I and then between the lugs 23 and overthe rim of the cap 22 but no matter at what angle the loud speaker ismounted rain water cannot have access to the housing I3.

In the above described apparatus the magnet is preferably made of one orother of the alloys known to be used for making such magnets from suchalloys as they permit of a large reduction of weight without efliciencyloss.

It will be appreciated that the invention may be applied not only topressure type loud speakers but also to all moving coil types and todynamic microphones.

I claim:

1. A loud speaker or the like of the moving coil type, comprising adiaphragm, an adjacent speech-current-carrying coil for vibrating saiddiaphragm, radially spaced and concentric inner and outer pole piecesproviding an annular air gap space therebetween, the outer pole piececomprising a circular plate having a hole into which the inner polepiece projects, an outer housing having a front opening, a front closureplate detachably secured to said housing for closing said front opening,said front closure plate having a circular recess in its rear surface,said diaphragm and outer pole piece being secured in said recess in saidfront closure plate.

2. A loud speaker or the like according to claim 1, wherein saiddiaphragm unit comprises the diaphragm proper and a flat ring framingtherefor of some stiffer material which ts very accurately in saidcircular recess and said circular plate ts accurately in said recessbehind said diaphragm and thereby clamps said framing in place.

3. A loud speaker or the like according to claim l, wherein saiddiaphragm unit comprises the diaphragm proper and a fiat ring framingtherefor of some stiier material which ts very accurately in saidcircular recess and said circular plate ts accurately in said recessbehind said diaphragm and thereby clamps said framing in place, andwherein said framing is slightly thicker than said diaphragm proper.

4. A loud speaker or the like according to claim 1 and wherein the innerpole piece is centred relative to said circular plate by means of anonferrous bush mounted on the back of said circular plate, in whichbush the rearwardly extending part of said inner pole piece veryaccurately nts, said bush having a plurality of holes in is rim and aplurality of cut-away portions along its inner edge.

LEONARD WALTER MURKHAM.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,766,473 Wente June 24, 19301,847,702 Thuras Mar. 1, 1932 2,104,433 Marshall Jan. 4, 1938 2,126,566Marshall Aug. 9, 1933 2,295,483 Knowles Sept. 8, 1942 2,403,535 KremerJuly 9, 1946 FOREIGN PATENTS Number Country Date 490,750 Great BritainAug. 19, 1938

